The present invention relates to microwave circuitry and more particularly to an air-stripline microwave coupler.
Microwave couplers are well known in the art and have been constructed in various configurations generally known as microstrip, stripline, coaxial, and wave guide couplers. Each of the known couplers have their own characteristics and are used in a variety of applications to optimize microwave transmission and coupling. In recent years, there has been an increase in the demand for devices which generate microwave energy over various frequency bands and at higher power levels than previously necessary. As the demand for high power devices has increased, so too has the demand for more efficient, reliable, low cost and compatible transmission lines and couplers. IMPATT diodes, for example, are extensively used for their power generating capabilities in the microwave regions for use in radar and communications systems, and new coupling devices are now required to more effectively use such IMPATT devices.
Naturally, in any power transmission or coupling circuit, it is desirable to couple or transmit energy with as little loss as possible. In many known techniques, coupling circuits exhibit high insertion loss, low isolation, high VSWR or narrow frequency operation, all of which cause degraded device performance. Depending on the specific configuration of the device, improved operation is normally attained only at the expense of increased cost and complexity. Such devices have thus been useful under limited circumstances but have failed to be widely accepted for use in diverse environments. In microwave stripline technology in particular, commercially available hybrid couplers have included structures which enclose the coupling lines with dielectric material. As the amount of dielectric is increased, greater isolation is achieved but higher insertion losses are experienced. A reduction in the amount of dielectric improves insertion loss but reduces the isolation caused by mismatch in even-odd mode phase velocities. In any event, the operation of such devices over wide frequency ranges and large temperature variations could not be achieved without a tradeoff between insertion loss and isolation.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an air-stripline microwave coupler having improved coupling and operational characteristics.